Recently, batteries having a light weight, a long service life and a high energy density are demanded as a main power source or an auxiliary power source for electric automobiles and fuel cell vehicles, or as a power source for small-size electronics devices. For this demand, a non-aqueous electrolyte battery using lithium as an active substance for a negative electrode is known as one of the batteries having a high energy density because an electrode potential of lithium is the lowest among metals and an electric capacity per unit volume is large, and many kinds of such a battery are actively studied irrespectively of primary battery and secondary battery, and a part thereof is practiced and supplied to markets. For example, the non-aqueous electrolyte primary batteries are used as a power source for cameras, electronic watches and various memory backups. Also, the non-aqueous electrolyte secondary batteries are used as a driving power source for note-type personal computers, mobile phones and the like, and further they are investigated to use as the main power source or the auxiliary power source for the electric automobiles and the fuel cell vehicles.
In these non-aqueous electrolyte batteries, since lithium as an active substance for a negative electrode violently reacts with a compound having an active proton such as water, alcohol or the like, an electrolyte used in these batteries is limited to an aprotic organic solvent such as ester compound, ether compound or the like. Although the aprotic organic solvent is low in the reactivity with lithium as the active substance for the negative electrode, there is a high risk that if a large current flows violently, for example, in the short-circuiting or the like and the battery generates abnormal heat, the aprotic organic solvent is vaporized and decomposed to generate a gas, or the generated gas and heat cause explosion and ignition of the battery, fire is caught by a spark generated in the short-circuiting or the like.
Moreover, an electric double layer capacitor is a condenser utilizing an electric double layer formed between an electrode and an electrolyte, in which a cycle of electrically adsorbing an ion on a surface of the electrode from the electrolyte is a charge-discharge cycle, so that it is different from a battery in which a cycle of oxidation-reduction reaction accompanied with a mass transfer is a charge-discharge cycle. Therefore, the electric double layer capacitor is excellent in the instant charge-discharge characteristics as compared with the battery, and further it has many merits that the instant charge-discharge characteristics are not substantially deteriorated even in the repetition of the charge-discharge because it is not accompanied with the chemical reaction, a simple and cheap electric circuit is sufficient in the electric double layer capacitor because there is no overvoltage in the charge-discharge, the residual capacity is easily understandable, the temperature durability is good over a wide temperature range of −30 to 90° C. and there is no pollution and the like, as compared with the battery. Therefore, the electric double layer capacitor is widely studied as a lower-capacity type for a memory backup or the like, a medium-capacity type for assisting a power of an electric automobile or the like and a higher-capacity type as a substitute for an accumulator battery for electric power storage or the like.
In a contact interface between the electrode and the electrolyte of the electric double layer capacitor, positive and negative charges are oppositely arranged at an interval of a very short distance to form an electric double layer. Therefore, the electrolyte plays a role as an ion source for the formation of the electric double layer, so that it is an important substance dominating the basic characteristics of the electric double layer capacitor likewise the electrode. As the electrolyte, there have hitherto been known an aqueous electrolyte, a non-aqueous electrolyte, a solid electrolyte and the like.
On the other hand, since a report of Wilkes et al. in 1992, an ionic liquid attracts attention as a substance which is a liquid at normal temperature and excellent in an ion conductivity. In the ionic liquid, a cation is bonded with an anion via an electrostatic attraction, the number of the ion carrier is very large, a viscosity is relatively low and thereby an ion mobility is high even at normal temperature, so that the ionic liquid has a very high ion conductivity. Further, the ionic liquid has a high boiling point and a very wide temperature range in which it can remain in a liquid state, because it consists of the cation and anion. Furthermore, the ionic liquid is low in a flammability and very excellent in a thermal stability, because it has little vapor pressure (see J. Electrochem. Soc., 144 (1997) 3881 and “Function Creation and Application of Tonic Liquid”, N. T. S, (2004)). The ionic liquid has these various merits, so that it is recently studied to apply to electrolytes for a non-aqueous electrolyte secondary battery and an electric double layer capacitor (see JP-A-2004-111294 and JP-A-2004-146346). In particular, when the ionic liquid is used in the electrolyte for the electric double layer capacitor, it also serves as an ion source for forming an electric double layer, so that there is also a merit that it is not necessary to add an additional supporting electrolyte.